Вы должны использовать метод конфигурации .AssignsOutAndRefParameters:
[Test]
public void Output_and_reference_parameters_can_be_configured()
{
var fake = A.Fake<IDictionary<string, string>>();
string ignored = null;
A.CallTo(() => fake.TryGetValue("test", out ignored))
.Returns(true)
.AssignsOutAndRefParameters("foo");
// This would of course be within you SUT.
string outputValue = null;
fake.TryGetValue("test", out outputValue);
Assert.That(outputValue, Is.EqualTo("foo"));
}